Nancy Reagan and two Santa Clauses, one of them undoubtedly an imposter, officially opened the White House holiday season yesterday.

Actor Larry Hagman -- alias J.R. Ewing from "Dallas" -- turned up in a fur-trimmed red suit and white beard at the morning press preview of the mansion's turn-of-the-century Christmas decor.

"There've been bigger Santas than I," he said, eyeing the fireplace in the Blue Room where the focal point was a 20-foot Fraser fir.

That became apparent a few hours later when he was succeeded by NBC "Today" show weatherman Willard Scott, almost half again as large in the familiar North Pole garb. Scott acted as master of ceremonies at the annual party for 350 children of diplomats from 80 countries. Sources said this year's turnout included a larger-than-usual contingent of 11 Soviet youngsters, indicative perhaps of a postsummit holiday spirit.

"Welcome to the best Christmas party in the world," Santa (Scott) told them. Volunteers from The Hospitality and Information Service (THIS), an organization that helps resident foreign diplomats, assisted with the celebration.

With Mrs. Reagan among them, the 6- to 10-year-olds, many in their national dress, sat on the floor of the East Room. They were entertained by giant Cabbage Patch dolls and Mickey and Minnie Mouse. On stage was Avner the Eccentric, a mime, and Emmanuel Lewis, star of the television series "Webster," who led them in a chorus of "Deck the Halls."

At both affairs, the first lady wore a red wool dress and black patent leather pumps with red pompons. At the morning session for the press, she led in her Christmas present, the white-and-brown King Charles spaniel named Rex that President Reagan gave her last week.

Hagman told reporters he had always wanted to play Santa Claus.

"You know, with the role I'm playing, the meanest guy in the world, people ask me what I want to play in the future -- if there is a future. I said Santa Claus. So Nancy called up about three weeks ago and asked if I wanted to -- it was fortuitous," Hagman said.

The White House state rooms -- decorated over the weekend by volunteer florists and residents of Second Genesis, a drug rehabilitation center -- were a profusion of mixed Christmas greens, wreaths with red bows, poinsettia plants, and spruce and fir trees.

The Blue Room tree, from Ash County, North Carolina, touched the ceiling and shimmered with a thousand miniature lights among 1,500 ornaments made from a selection of the 40,000 Christmas cards the Reagans received last year. Second Genesis volunteers started making the ornaments in August.

Under the tree were two wooden trains and a depot built by White House craftsmen. One engine was named "Big Rex" after chief usher Rex Scouten, who is retiring soon.

In the East Room were six large blue spruce trees decorated with tiny lights, tinsel icicles and simulated snow. On all the mantels were dolls wearing turn-of-the-century costumes designed by White House flower shop personnel.

"Every Christmas is special for me," Mrs. Reagan said of her fifth one in the White House. "I always think it looks pretty. And I always think it's the prettiest."

She said she already has her husband's Christmas present, but refused to say what it is. Was she going to give him Donald Regan, the controversial White House chief of staff? asked one reporter, hoping to liven the party.

"Naughty, naughty," Mrs. Reagan scolded, laughing. "It's Christmas, the season of good will. Bah, humbug!"